CSME Heat Transfer and Energy Systems Seminar Series

Space cooling consumes significant amount of power and contributes to urban heat island effect and global warming. Radiative cooling, a passive cooling method that reflects solar irradiation and emits infrared heat, both to deep space, has the promise of not only saving energy but also directly cooling down the earth. In this seminar, I will describe our invention of ultrawhite paints that show remarkable below-ambient cooling under direct sunlight, the physics behind it, and the implications for energy savings and climate crisis mitigation. We have fabricated BaSO4-acrylic and CaCO3-acrylic paints that reflect 98.1% and 95.5% of sunlight respectively (only absorb 1.9% and 4.5% respectively). The sky window emissivity is 0.94-0.96. These optical properties enable that our paints absorb less heat from the sun than the infrared heat they emit, hence cooling below the ambient temperature under direct sunlight. In particular, field tests show that the BaSO4 paint stays more than 4.5 °C below ambient temperature and achieves an average cooling power of 117 W/m^2. We have performed first principles and photon Monte Carlo simulations to guide or explain the materials design, and the results show that this high performance is enabled by synergistically pushing a few factors to the extremes. Unlike TiO2 that is widely used in commercial white paints, BaSO4 and CaCO3 have wider electronic band gaps that eliminate the absorption of ultraviolet photons in the solar irradiation, while the abundant vibrational modes in acrylic and BaSO4 as well as strong four-phonon scattering in BaSO4 offer high emissivity in the sky window. Meanwhile, the appropriate range of particle size similar to solar photon wavelength, a broad particle size distribution, and high particle concentration all have essential contributions to the ultra-high solar reflectance. Finally, we model the applications of the paints as pre-cooling unit for traditional air conditioners or directly on the building envelopes, and show significant potential for energy savings and climate crisis mitigation.

Dr. Xiulin Ruan is a professor in the School of Mechanical Engineering and Birck Nanotechnology Center at Purdue University. He received his B.S. and M.S. from the Department of Engineering Mechanics at Tsinghua University, in 2000 and 2002 respectively. He then received an M.S. in electrical engineering in 2006 and Ph.D. in mechanical engineering in 2007 from the University of Michigan at Ann Arbor, before joining Purdue. His research and teaching interests are in predictive simulations, scalable manufacturing, and multiscale characterizations of thermal transport materials and systems. Notably, his recent fundamental work has established a new research topic on four-phonon and higher-order phonon scattering, and his recent applied work on ultrawhite radiative cooling paints has appeared in more than 2,000 news reports globally and earned a Guinness World Record. He received the NSF CAREER Award, Air Force Summer Faculty Fellowship, ASME Heat Transfer Division Best Paper Award, the inaugural School of Mechanical Engineering Outstanding Graduate Student Mentor Award, and was named a University Faculty Scholar of Purdue University and an ASME Fellow, among his honors. He currently serves as an associate editor for ASME Journal of Heat Transfer.